1. Field of the Invention
The present invention relates to communications; more specifically, message encoding.
2. Description of the Related Art
FIG. 1 illustrates a typical cellular based wireless communication system. Communications from base station 10 to mobile stations 12, 14 and 16 are transmitted using downlink channels 18, 20 and 22, respectively. Similarly, communications from mobile stations 12, 14 and 16 to base station 10 are transmitted using uplink channels 24, 26 and 28, respectively.
FIG. 2 illustrates a downlink data communication channel having time slots 40, 42, 44 and 46. The data transmitted in each of the slots may be transmitted at one of several standard rates. For example, slot 40 contains data transmitted at rate R1, slot 42 has data transmitted at rate R2, slot 44 has data transmitted at rate R5 and slot 46 has data transmitted at rate R12. The rate used to transmit data in each slot is chosen based on a request made by the mobile station that will be receiving the data. For example, while a mobile is receiving data at rate R1 during time slot 40, the mobile is also monitoring communication channel conditions by measuring, for example, a carrier to interference ratio. Based on that measurement, the mobile station transmits a data rate request to base station 10 before the end of slot 40 so that the base station can transmit at the new rate during slot 42. If, for example, the carrier to interference ratio has increased indicating improving channel conditions, the mobile will request rate R2 for the next time slot where rate R2 is a higher data rate than rate R1. Similarly, if during slot 42 the carrier to interference ratio improves significantly, the mobile may ask for a rate increase to rate R5 for slot 44. The number of bits used to specify the new rate depends on the number of standard rates that are available. In a proposed communication system known as HDR (High Data Rate), 14 different data rates are available, therefore the request requires four bits.
FIG. 3 illustrates an uplink control channel known as the pilot/DRC (data rate control channel). The pilot/DRC channel is transmitted by the mobile to provide the base station with a pilot signal that the base station uses to demodulate other transmissions from the mobile to the base station. The pilot/DRC is also used to provide the base station with data rate request information from the mobile. The pilot and data rate request information are alternated using equal amounts of time. For example, time slot 60 is used to transmit pilot signal while time slot 62 is used to transmit bit B1 of a data rate request message. Slot 64 is then used to transmit pilot information while slot 66 is used to transmit bit B2 of the data rate request message. Slot 68 is used to transmit pilot signal, and slot 70 is used to transmit bit B3 of the data rate request message. Similarly, slot 72 is used to transmit pilot signal while slot 74 is used to transmit bit B4 of the data rate request message. This pattern repeats until another data rate request message is sent where bits B1, B2, B3 and B4 of prior data rate request message are replaced with four bits specifying the new rate.
The height of each slot indicates the power level used to transmit the information in the slot. It should be noted that the pilot and data rate request bits are transmitted at the same power level. As a result, 50% of the power in the pilot/DRC channel is associated with data rate request messages. Generally, any overhead or control channel such as a pilot/DRC channel increases the overall noise in a communication system and thereby adversely affects the quality of the voice or data communication channels. Therefore, it is desirable to reduce the amount of power in overhead or control channels.